说明：  北京大学ACM比赛题 The game of Scrabble is played with tiles. A tile either has a single letter written on it, or it is blank. In the latter case, the tile may be used to represent a letter of your choice. On your turn, you arrange the tiles to form a word. Each tile may be used at most once, but not all tiles need to be used. Given several Scrabble tiles and a dictionary, determine how many words in the dictionary can be formed using the given Scrabble tiles.
(ACM Peking title game The game of Scrabble is played with tiles. A tile either has a single letter written on it, or it is blank. In the latter case, the tile may be used to represent a letter of your choice. On your turn, you arrange the tiles to form a word. Each tile may be used at most once, but not all tiles need to be used. Given several Scrabble tiles and a dictionary, determine how many words in the dictionary can be formed using the given Scrabble tiles.)